Frequency and spatial distribution of landslides in a mountainous drainage basin: Western Foothills, Taiwan

Maps from 1904 and 1915 and air photographs from 1963, 1980, 1985, 1993 and 1996 provide a record of landslide incidence in a 92.1-km(2) drainage basin, a headwater tributary of the Cho-Shui River in Taiwan. Interpretation of landslide patterns from the early maps indicate that in four sub-basins (36 km(2)) structural geological factors control chronic landsliding regularly reactivated by intense rains. Within these four sub-basins, all later air photographs reveal a continuing high incidence of landslides (with landslide densities of 3 - 11 ha/km(2)). Air photographs taken in 1963, following extensive logging, in 1985, following highway construction, and in 1996, following the very large typhoon Herb event demonstrate the short-term effects of disturbance in these structurally weak sub-basins. Air photographs from 1980 and 1993 demonstrate recovery of the land surface from logging and highway construction impacts, respectively. For the adjacent sub-basins (56 km 2), two modes of response to perturbations were identified: six sub-basins (48 km(2)) showed direct response to logging, road construction and typhoon Herb and five sub-basins (8 km(2)) were more buffered and showed some lagged responses. Even this last category of sub-basins is more active than the average for Taiwan, where the mean landslide density is 0.84 ha/ km(2). It is proposed that, for the 92.1-km(2) Hoshe basin, the 'formative event' sensu Brunsden [Z. Geomorphol. Suppl. 79 (1990) 1] is one that produces approximately 200 ha of landslides, a value that has been equaled or exceeded in each of the periods of study (1963-1980, 1980-1985, 1985 - 1993 and 1993 - 1996). Logging activity, major road construction, and extreme typhoon and earthquake events produce short-term acceleration of landslide incidence. In principle, recovery rates of the land from pulsed perturbations of about 20 years for logging activity and about 8 years for major road construction may also be suggested. (C) 2002 Elsevier Science B.V. All rights reserved.